Classification of compactified $su(N_c)$ gauge theories with fermions in all representations

TL;DR

This paper classifies $su(N_c)$ gauge theories with massless fermions in various representations on $R^3 imes S^1$, identifying those that are asymptotically free, weakly coupled, and exhibit phenomena like spontaneous symmetry breaking and confinement.

Contribution

It provides a comprehensive classification of $su(N_c)$ gauge theories with fermions in all representations, highlighting new examples of weakly coupled theories with spontaneous symmetry breaking of discrete symmetries.

Findings

Identified asymptotically free, weakly coupled theories with mixed fermion representations.

Discovered theories with degenerate vacua that break ${ m C}$, ${ m P}$, and ${ m T}$ symmetries.

Analyzed monopole-instantons and topological molecules leading to confinement.

Abstract

We classify $su(N_c)$ gauge theories on $\mathbb R^3\times \mathbb S^1$ with massless fermions in higher representations obeying periodic boundary conditions along $\mathbb S^1$. In particular, we single out the class of theories that is asymptotically free and weakly coupled in the infrared, and therefore, is amenable to semi-classical treatment. Our study is conducted by carefully identifying the vacua inside the affine Weyl chamber using Verma bases and Frobenius formula techniques. Theories with fermions in pure representations are generally strongly coupled. The only exceptions are the four-index symmetric representation of $su(2)$ and adjoint representation of $su(N_c)$. However, we find a plethora of admissible theories with fermions in mixed representations. A sub-class of these theories have degenerate perturbative vacua separated by domain walls. In particular, $su(N_c)$ theories with fermions in the mixed representations adjoint$\oplus$fundamental and adjoint$\oplus$two-index symmetric admit degenerate vacua that spontaneously break the parity ${\cal P}$, charge conjugation ${\cal C}$, and time reversal ${\cal T}$ symmetries. These are the first examples of strictly weakly coupled gauge theories on $\mathbb R^3 \times \mathbb S^1$ with spontaneously broken ${\cal C}$, ${\cal P}$, and ${\cal T}$ symmetries. We also compute the fermion zero modes in the background of monopole-instantons. The monopoles and their composites (topological molecules) proliferate in the vacuum leading to the confinement of electric charges. Interestingly enough, some theories have also accidental degenerate vacua, which are not related by any symmetry. These vacua admit different numbers of fermionic zero modes, and hence, different kinds of topological molecules. The lack of symmetry, however, indicates that such degeneracy might be lifted by higher order corrections.

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